Anion Responsive Molecular Switch Based on a Doubly Strapped Calix[4]pyrrole

Abstract

A calix[4]pyrrole receptor bearing two proximally meso-meso linking isophthaloyl straps displays open and closed states depending on the calix[4]pyrrole conformation. In the crystal structures and in non-polar solvent, the calix[4]pyrrole adopts open 1,3-alternate conformation with straps on the sides. Anion binding triggers a closed state of the receptor providing two types of interactions with an aromatic benzoate guest: hydrogen bonds from the pyrrolic NH groups and π⋅⋅⋅π interactions from the phenyl groups of the straps. Slow exchange dynamics was observed on the NMR timescale indicating that benzoate, acetate and chloride anions, which bind with relatively low affinity get kinetically trapped in the receptor as insects in a Venus flytrap.